Solubility of lignin in microemulsion system
However, the solubilization effect is
one of the most important features of the microemulsion system,
therefore the solubilities of lignin in different microemulsions were
measured and listed in Table 1. The lignin sample originated from
bagasse is insoluble in both water and octane (Table 1, entries 1 and
2), but it can dissolve in n -propanol with the solubility of 19.9
g L-1 (Table 1, entry 3). Basing on the above
constructed SFME system and the recognition of its subregion, the
solubilities of lignin in different microemulsions were investigated. At
first, different O/W microemulsions at a fixed RP/Ovalue of 8.16 were selected to test the effect of water content (Table
1, entries 4-8), considering that water is a kind of green and easily
available solvent. The solubility of lignin in then -propanol/octane system without water (Table 1, entry 4, point a
in Figure 3) is close to that in n -propanol. However, due to the
solubilization effect and the changing polarity of microemulsions, the
solubility of lignin is enhanced significantly by adding water to then -propanol/octane system. For example, when the
WW value increases from 7.6 to 15.7 wt. %, the
solubility of lignin sharply rises from 28.6 to 79.1 g
L-1 (Table 1, entries 5 and 6, points b and c in
Figure 3), which is compatible to that obtained in ionic
liquids.37,38 And it remains about 71.9 g
L-1 when the WW value further
increases to 31.2 wt. % (Table 1, entries 7 and 8, points d and e
in Figure 3). It implies that suitable polarity for the microemulsion
may act as an important parameter to affect the solubility of lignin,
because the polarity of the microemulsion increases with the elevated
WW value as illustrated in Figure S2A, while lots of
polar groups in lignin macromolecules also provide contribution to the
solubilization. In order to give a more understanding about the
solubilization effect in all subregions, the solubility of lignin
affected by n -propanol content (WP) was further
determined under the fixed mass ratio of water to octane
(RW/O) basing on the maximum value obtained above (Table
1, entries 6, 9 and 10, points c, f and g in Figure 3). The solubility
of lignin sharply increases at first and then decreases slightly with
the increase of WP, and it reaches the maximum value at
the WP value of 75.1 wt. %, which is the same
point as mentioned above. Besides, it is worth noticing that the
solubilities of lignin in the most given O/W microemulsions improve
greatly, compared with those in B.C and W/O microemulsions (Table 1,
entries 9 and 11, points f and j in Figure 3). More interestingly, all
the solubilities of lignin in the given microemulsions are larger than
that in n -propanol or n -propanol/octane binary system
under the same conditions (Table 1, entries 5-11 vs 3 and 4),
this does verify enhanced solubilization of the microemulsions.
Therefore, the microemulsion possesses the best solubility for lignin is
regarded largely with the composition of 15.7 wt. % water, 9.2wt. % octane and 75.1 wt. % n -propanol, exhibiting
about four times of the lignin solubility in comparison to that inn -propanol.
Table 1 . Solubility of lignin in different microemulsions